Conventionally, variable capacitance devices have been used whose capacitance is changed by application of external bias signals to thereby control the frequency, time period, or the like of an input signal. Such variable capacitance devices have been made commercially available as, for example, variable capacitance diodes (varicaps) and MEMS (Micro Electro Mechanical Systems).
In addition, conventionally, there has been proposed a technique for using the above-described variable capacitance device as a protection circuit in a non-contact IC (Integrated Circuit) card (see, for example, PTL 1). According to the technique described in PTL 1, a variable capacitance device is used as a protection circuit in order to prevent a control circuit made of a semiconductor device with a low withstand voltage from being destroyed by an excessively large received signal when a non-contact IC card is brought closer to its reader/writer.
FIG. 60 is a block diagram of the non-contact IC card proposed in PTL 1. In PTL 1, a variable capacitance diode 303d is used as a variable capacitance device. A series circuit of a bias removal capacitor 303c and the variable capacitance diode 303d is connected in parallel to a resonance circuit including a coil 303a and a capacitor 303b. 
In PTL 1, a DC voltage Vout obtained by detecting a received signal in a detector circuit 313 is subjected to resistive division between resistors 314a and 314b. Then, the DC voltage after the resistive division (DC voltage applied across the resistor 314b) is applied to the variable capacitance diode 303d via a coil 315 provided to eliminate fluctuations in the DC voltage, thereby adjusting the capacitance of the variable capacitance diode 303d. That is, the DC voltage that has been subjected to resistive division is used as a control voltage for the variable capacitance diode 303d. 
In PTL 1, when the received signal is excessively large, the capacitance of the variable capacitance diode 303d becomes small due to the control voltage, so the resonance frequency of a reception antenna 303 becomes high. As a result, the response of the received signal at resonance frequency f0 of the reception antenna 303 before change in capacitance becomes lower than that prior to the decrease in capacitance, thereby enabling suppression of the received signal level. According to the technique proposed in PTL 1, a signal processing unit 320 (control circuit) is protected by the variable capacitance device in this way.
The present inventors have also proposed a device using a ferroelectric material as a variable capacitance device (see, for example, PTL 2). PTL 2 proposes a variable capacitance device 400 having an electrode structure as shown in FIG. 61(A) and FIG. 61(B) to achieve an improvement in reliability and productivity. FIG. 61(A) is a schematic perspective view of the variable capacitance device 400, and FIG. 61(B) is a cross-sectional diagram of the variable capacitance device 400. In the variable capacitance device 400 according to PTL 2, a terminal is provided in each of the four faces of a rectangular parallelepiped dielectric layer 404. Of the four terminals, two opposing terminals on one side are signal terminals 403a and 403b connected to a signal power supply 403, and two opposing terminals on the other side are control terminals 402a and 402b connected to a control power supply 402.
As shown in FIG. 61(B), the internal structure of the variable capacitance device 400 is such that a plurality of control electrodes 402c to 402g and a plurality of signal electrodes 403c to 403f are layered alternately via the dielectric layer 404. Specifically, from the bottom layer, a control electrode 402g, signal electrode 403f, control electrode 402f, signal electrode 403e, control electrode 402e, signal electrode 403d, control electrode 402d, signal electrode 403c, and control electrode 402c are layered in this order via the dielectric layer 404. In the example shown in FIG. 61(B), the control electrode 402g, the control electrode 402e, and the control electrode 402c are connected to the control terminal 402a, the control electrode 402f and the control electrode 402d are connected to the other control terminal 402b, and the signal electrode 403f and the signal electrode 403d are connected to the signal terminal 403a. Also, the signal electrode 403e and the signal electrode 403c are connected to the other signal terminal 403b. 
In the case of the variable capacitance device 400 according to PTL 2, voltages can be individually applied to the control terminals and signal terminals, and a plurality of signal electrodes and control electrodes are layered inside the variable capacitance device 400, which advantageously enables increasing capacitance at low cost. In addition, the variable capacitance device 400 having a structure as described in PTL 2 can be manufactured easily and is low in cost. Further, no bias removal capacitor is necessary in the case of the variable capacitance device 400 according to PTL 2.